Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
U.S. Pat. No. 3,031,329 discloses a stabilized heat-sensitive copy-sheet for making a clear and sharp reproduction of a graphic original by a thermographic copying procedure involving brief application of a heat-pattern corresponding to said original, said copy-sheet being visibly stable under normal storage conditions and when subjected to elevated temperatures up to about 60° C. for limited periods while being rapidly permanently visibly changed on heating to a conversion temperature of the order of about 90-150° C., said copy-sheet including a visibly heat-sensitive layer containing, in intimate association, a normally solid organic acid salt of a noble metal, and a cyclic organic reducing agent for the noble metal ions, said reducing agent having an active hydrogen atom attached to an atom which is selected from the class of oxygen, nitrogen and carbon atoms and which is directly attached to an atom of the cyclic ring, said reducing agent being capable of causing reduction of silver ions and precipitation of metallic silver on being dissolved at moderate temperature in a solution of aqueous silver nitrate in an organic solvent, and said visibly heat-sensitive layer being characterized by the inclusion of a significant small amount of a perhalogenated aromatic organic acidic stabilizer material sufficient to improve greatly the stability of the heat-sensitive layer. Tetrachlorophthalic acidic material and tetrachlorophthalic anhydride are specifically claimed in U.S. Pat. No. 3,031,329, with the use of tetrachlorophthalic anhydride being specifically disclosed and exemplified and tetrabromophthalic acid and tetrachlorophthalic acid being also specifically disclosed.
U.S. Pat. No. 3,911,171 discloses a thermographic recording process comprising producing a dye image by bringing into image-wise reactive contact with the aid of heat a dye precursor compound and a generally non-crystallizing compound with acid reaction corresponding to the following general formula:
wherein Z represents atoms to close an unsubstituted nucleus, a halogen-substituted nucleus or a nitro-substituted benzene nucleus, and R represents an unsubstituted aliphatic group containing at least 4 carbon atoms, a cycloaliphatic group, or an aliphatic group substituted with hydroxy, with an etherified hydroxyl group or with an acyloxy group.
U.S. Pat. No. 3,965,282 discloses a mono-sheet thermosensitive recording material containing a dye precursor compound and a compound with acid reaction, in which material the compound with acid reaction and the dry precursor compound at a temperature below 60° C. are kept out of direct chemical contact and the compound with acid reaction corresponds to the following general formula:
wherein Z represents atoms to close an unsubstituted nucleus, a halogen-substituted nucleus or a nitro-substituted benzene nucleus, and R represents an unsubstituted aliphatic group containing at least 4 carbon atoms, a cycloaliphatic group or an aliphatic group substituted with hydroxy, with an etherified hydroxyl group or with an acyloxy group.
U.S. Pat. No. 3,911,171 and U.S. Pat. No. 3,965,282 specifically disclose the following mono-esters of tetrachlorophthalic acid: isobutyl, 2-hydroxyethyl, 3-hydroxybutyl, 2,2-dimethyl-3-hydroxy-propyl, n-butyl, 2-ethylbutyl, 2-methylpentyl, 2-methoxy-ethyl, cyclohexyl, 1-methylpentyl, decyl and 9-vinyl-nonyl.
U.S. Pat. No. 4,011,352 discloses a thermographic recording process in which a dye is produced by bringing image-wise into reactive contact with the aid of heat an organic acid-reacting compound with a dye precursor compound, the improvement being producing a light stable yellow image by bringing said compound into contact with a dye precursor corresponding to the following general formula:X—Ar—CH═N—R1wherein: Ar represents a phenylene nucleus, R1 represents a phenyl group and X represents a
group wherein each of R2 and R3 (same or different) represents an alkyl group, a cyanoalkyl group, a chloroalkyl group, and an alkoxy carbonylalkyl group. U.S. Pat. No. 4,011,352 specifically discloses the monomethyl, monoethyl and monoisopropyl tetrachlorophthalates.
U.S. Pat. No. 4,315,068 discloses a photo-sensitive and heat-sensitive composition which comprises (a) a photooxidant, (b) a color-generator generating color when oxidized, (c) an acid promoting said color generation, (d) a cobalt (III) amine and/or amine complex, (e) a photoreductant, (f) a hydrogen donator, (g) at least one kind of chelating agent selected from dioxime chelating agents having the following general formula (I):
(wherein n is an integer of 0 to 3, and R1 and R2 can each represent an alkyl, aryl or aralkyl group) and diantipyrylmethane chelating agents having the following general formula (II):
(wherein R3, R4, R5, R3′, R4′ and R5′ can each represent a hydrogen atom and an alkyl or aryl group, and R6 and R7 can each represent a hydrogen atom and an alkyl, substituted or non-substituted aryl or aralkyl group), and (h) at least one kind of stabilizer selected from noble metal complexes, organic oxidants and organic acids. Among the organic oxidants disclosed are compounds having the general formula (7):
(wherein, X5 can represent chlorine or bromine, and R5 and R6 can represent C1-C12 straight chain or branched alkyl groups). As their exemplary examples there can be enumerated tetrachlorophthalic acid, monomethyl tetrachlorophthalate, diethyl tetrachlorophthalate, dioctyl tetrachlorophthalate and the like.
EP-A 0 097 615 discloses a process for preparing particular cyanobenzoic acid esters and discloses the use of the monomethyl tetrachlorophthalate as a starting material in this process.
U.S. Pat. No. 3,911,171, U.S. Pat. No. 3,965,282, U.S. Pat. No. 4,011,352 and U.S. Pat. No. 4,315,068 all disclose the use of mono-alkyl and/or dialkyl tetrachlorophthalates in heat- and photo-sensitive imaging elements, but as organic oxidants for use with a dye precursor rather than in association with substantially light-insensitive organic silver salts and reducing agents therefor.
Substantially light-insensitive thermographic recording materials based on substantially light-insensitive organic silver salts contain the imaging-forming components both before and after image formation and unwanted image-forming must be hindered during storage prior to printing, in prints exposed to light on light-boxes e.g. during examination by radiologists, and during storage of prints under dark conditions. Furthermore, such stabilization must take place without adverse effects upon the image quality particularly the image tone and Dmax. Stabilizers for substantially light-insensitive thermographic recording materials based on substantially light-insensitive organic silver salts, such as tetrachlorophthalic anhydride, tetrachlorophthalic acid and benzotriazole, are known, which realize acceptable image stabilization upon archival and upon exposure to light with acceptable deterioration in image tone. However, such stabilizers decrease Dmax, a decrease which is substantial at the concentrations required for realizing acceptable image stabilization. There is therefore a continuing need for new stabilizers for substantially light-insensitive thermographic recording materials based on substantially light-insensitive organic silver salts which provide image stabilization without adverse effects upon image tone and Dmax.